U.S. Pat. No. 4,585,067 discloses a choke and kill valve which is initially latched in the open position by a piston and cylinder assembly. However, that assembly is actuated by the difference between the pressure of a control fluid and the pressure in the well tubing. Since the control fluid extending to the well surface includes hydrostatic pressure, the assembly is subject to inadvertently closing the choke and kill valve in the event that the tubing pressure decreases. Another disadvantage of the prior art system is that once it is set in place the closing force cannot be changed to meet changing well conditions without pulling the entire tubing string. In addition, such a valve does not have a positive lockout which is desirable when using workover tools in the well. Furthermore, the hydraulic piston and cylinder assembly is subject to well contamination which may interfere with its operation.
The present invention overcomes the disadvantages of the prior art by using a control system having a sidepocket mandrel having a flow control means which is insensitive to tubing pressure and can be retrieved and replaced for changing the actuating pressure and can be provided with a blanking plug for locking the kill valve in the open position. Additionally, the hydraulic piston and cylinder assembly can be flushed out and cleaned when desired.